1. Field of the Invention
The present invention relates to a manufacturing method of a member having a chamfered through hole having a chamfered portion in peripheral corners located at each end of the through hole.
2. Description of the Related Art
For example, in a carrier cover of an automobile automatic transmission (A/T) or other various mechanical parts, a chamfered through hole having a chamfered portion in peripheral corners located at each end of the through hole is sometimes provided in a plate-like portion of the part.
Such a chamfered through hole is provided by, for example, a method disclosed in Japanese Patent Application Publication No. JP-A-S58-125320.
First, as shown in FIG. 12, a prepared-hole punching step is performed to form a prepared hole 90 extending through a plate-like portion 9 of a part by using a die 80 having a cut hole 800 and a punch 81.
Next, as shown in FIG. 13, the rear surface of the plate-like portion is supported by a stage 82, and a chamfering punch 83 having a pressing surface 830 corresponding to a desired chamfering shape is driven into the prepared hole 90 from its one end to form a first chamfered portion 91. With formation of the first chamfered portion 91, a burr-like unnecessary portion 94 protruding radially inward is formed inside the prepared hole 90.
Next, as shown in FIG. 14, the rear surface of the plate-like portion 9 is supported by a die 84 having a cut hole 840, and a finishing punch 85 is inserted into the prepared hole from the first chamfered portion 91 side to punch out the unnecessary portion 94. A finished hole 93 is thus formed.
Then, as shown in FIG. 15, the plane of the finished hole 93 having the first chamfered portion 91 is supported by a stage 86, and a chamfering punch 87 having a pressing surface 870 corresponding to a desired chamfering shape is driven against the periphery of the opposite end of the finished hole 93 to form a second chamfered portion 92. Formation of the chamfered through hole is thus completed. Note that, in order to form the second chamfered portion 92, a mold structure is designed so that generation of an unnecessary portion extending inward of a hole is suppressed to an acceptable level. In order to finish the finished hole 93 with higher accuracy, a cutting process may be performed as required.
In the above known method, formation of the finished hole 93 by the finishing punch 84 needs to be performed between the two chamfered-portion forming steps. It is more advantageous for cost reduction of the manufacturing process to perform the two chamfered-portion forming steps successively or simultaneously. However, it is practically difficult. If punching by the finishing punch is performed after the two chamfered portions are formed, appropriate support cannot be obtained with a die as shown in a comparative example (see FIG. 11) to be explained later. As a result, a shearing position may extend and a required portion may also be removed. Moreover, a sheared shape may become rugged instead of being circular, which is aesthetically problematic.
These problems can be solved by directly supporting the chamfered portion by a conically protruding die having a cut hole in the center. However, this method is problematic in terms of the fabrication cost and durability of the die.